The invention relates to new stabilizers for recording sheets used in ink jet printing, containing porous inorganic oxides.
Ink jet printing processes are mainly of two types: continuous stream and drop-on-demand.
In continuous stream ink jet printing systems, ink is emitted in a continuous stream under pressure through a nozzle. The stream breaks up into droplets at a certain distance from the nozzle. If a specific location on the recording sheet has to be printed the individual droplets are directed to the recording sheet, otherwise they are directed to a gutter. This is done for example by charging unnecessary droplets in accordance with digital data signals and passing them through an electric static field which adjusts the trajectory of these droplets in order to direct them to the gutter. The inverse procedure may also be used wherein uncharged droplets are directed to the gutter.
In the non-continuous process, or the so-called xe2x80x9cdrop-on-demandxe2x80x9d systems, a droplet is generated in accordance with digital data signals only if it is to be placed onto the recording sheet.
The printing speed of modern ink jet printers is always increasing for economical reasons. Recording sheets that are suitable for these printers therefore need to absorb the inks very quickly. Especially suitable are recording sheets containing porous inorganic oxides.
Such recording sheets available today do not meet all of the required demands. In particular, the storage stability of images printed on these recording sheets has to be improved. These images are not particularly stable when they are in contact with ambient air, which normally contains sulfur dioxide and, especially in summer, photochemically generated impurities such as ozone or nitrogen oxides. The images are strongly altered or even destroyed in a short time when they are in contact with ambient air. These phenomena are described for example in Hardcopy Supplies Journal, 6 (7), 35 (2000).
In patent GB 2,088,777 derivatives of phenols and bisphenols are proposed in order to improve the stability of recording sheets containing inorganic oxides.
In patent application EP 0,685,345 the addition of dithiocarbamates, thiocyanates, thiurams or sterically hindered amines to recording sheets containing porous inorganic oxides is proposed in order to improve their stability.
In patent application EP 0,373,573 derivatives of polyhydroxybenzenes are proposed as stabilizers for recording sheets for ink jet printing.
All of these proposed additives only marginally increase the stability of recording sheets for ink jet printing containing porous inorganic oxides when they are in contact with ambient air.
Stabilizing additives for such recording sheets containing porous inorganic oxides need to be sufficiently soluble and compatible with the other ingredients of the mainly aqueous coating compositions. They need to be colorless or are allowed to be colored only slightly. Furthermore, these additives need to be stable when the recording sheets or the images printed thereon are stored over long periods and they are not allowed to yellow. Additionally they need to be non-toxic and inoffensive.
There is therefore an urgent need to improve, in addition to the ink absorption capacity, the image quality, the water fastness, the light stability etc., in particular the storage stability of recording sheets containing porous inorganic oxides when they are in contact with ambient air containing impurities such as ozone, nitrogen oxides or sulfur dioxide.
An objective of the invention is to provide recording sheets containing porous inorganic oxides with improved storage stability when in contact with ambient air, where images recorded thereon can be observed by both reflected and transmitted light, and consist of a support having coated thereon at least one ink-receiving layer.
We have surprisingly found that the storage stability of recording sheets according to the invention is considerably improved in contact with ambient air when substituted or unsubstituted 1,3-cyclohexanedione is added to the recording sheets. Images printed onto such recording sheets according to our invention show considerably less change of colors and/or dye losses when in contact with ambient air containing impurities such as ozone, nitrogen oxides or sulfur dioxide in comparison with images printed onto recording sheets not containing these compounds.
The recording sheets for ink jet printing according to the invention contain in the coated layers, besides the porous inorganic oxide and the substituted or unsubstituted 1,3-cyclohexanedione, one or more binders. The substituted or unsubstituted 1,3-cyclohexanedione may be added to the layer containing the inorganic porous oxide or to any other layer of the recording sheet.
Preferred additives are 1,3-cyclohexanedione and 2-methyl-1,3-cyclohexanedione.
Further compounds such as reducing agents, organic thio compounds and inorganic thiocyanates may be added to the recording sheets according to the invention.
Additionally a reduction catalyst may be added together with the substituted or unsubstituted 1,3-cyclohexanedione, the reducing agent and the organic thio compounds or inorganic thiocyanates.
The invention will be described in detail in the following description.
The invention describes a recording sheet for ink jet printing consisting of a support having coated thereon one or more layers comprising, besides a porous inorganic oxide and binders in the ink-receiving layer, substituted or unsubstituted 1,3-cyclohexanedione. The porous inorganic oxide and the substituted or unsubstituted 1,3-cyclohexanedione may be contained in the same or in different layers.
These 1,3-cyclohexanediones have already been proposed as heat stabilizers or as additives for polyvinyl chloride in order to prevent yellowing in U.S. Pat. No. 4,252,698 or in patent application JP 51-111,852.
An object of the present invention is the addition of tautomeric compounds of formulas Ia (diketo form) and Ib (enol form) to recording sheets for ink jet printing containing a porous inorganic oxide 
wherein in formula Ib (enol form)
M represents a hydrogen cation, a metal cation such as Li, Na or K, a triethanolamine cation or an ammonium cation optionally substituted by one or more alkyl or substituted alkyl groups each having from 1 to 18 C atoms;
R1 represents hydrogen, alkyl with 1 to 12 C atoms or substituted alkyl with 2 to 6 C atoms, wherein the substituents are selected from the group consisting of CN, COOH, OH and COOR4, where R4 represents alkyl with 1 to 12 C atoms; and
R2, R3 independently represent hydrogen, alkyl with 1 to 6 C atoms or substituted alkyl with 2 to 6 C atoms, wherein the substituents are selected from the group consisting of CN, COOH, OH and COOR5, where R5 represents alkyl with 1 to 12 C atoms.
Especially preferred are compounds of formula Ia and Ib, wherein in formula Ib (enol form)
M represents a hydrogen, a Li, a Na, a K, an ammonium or a triethanolamine cation;
R1 represents hydrogen, alkyl with 1 to 3 C atoms or alkyl substituted by COOR4 with 2 to 4 C atoms, where R4 represents alkyl with 1 or 2 C atoms; and
R2, R3 independently represent hydrogen, alkyl with 1 or 2 C atoms or alkyl with 2 to 4 C atoms substituted by COOR5, where R5 represents alkyl with 1 or 2 C atoms.
Preferred additives are 1,3-cyclohexanedione and 2-methyl-1,3-cyclohexanedione.
Such a recording sheet contains one or more of these compounds. The quantity is from 1 mg/m2 to 5,000 mg/m2, preferably from 50 mg/m2 to 600 mg/m2 of these compounds.
In addition to the compounds of formulas Ia and Ib reducing agents may be added to the recording sheet as for example ascorbic acid or benzenephosphonic acid.
In addition to the compounds of formulas Ia and Ib organic thio compounds such as thioglycols or thiocarboxylic acids, in particular thiodiethyleneglycol or thiodipropionic acid, or inorganic thiocyanates such as ammonium thiocyanate, may be added.
It is especially preferred if, in addition to the reducing agents, the organic thio compounds or the inorganic thiocyanates, a reduction catalyst such as mandelic acid is added.
Preferably, the compounds of formulas Ia and Ib are used together with phenolic compounds such as 2,6-dihydroxybenzoic acid.
Compounds of formulas Ia and Ib are partly known and in some cases commercially available or they may be prepared by known methods (as described for example in Houben-Weyl, Methoden der organischen Chemie, 4th edition, Georg Thieme Editions, Stuttgart, Volume 7/2b, Pages 1617 ff (1976)).
Especially suitable compounds of formulas Ia and Ib are the compounds of formulas (1), (2), (3), (4), (5) and (6) 
The additives according to the invention may added to the recording sheets in form of their aqueous, acid solutions or in form of their lithium, sodium or potassium salts, or in form of their salts with triethanolamine or their ammonium salts in neutral or basic solution. The ammonium salt may be substituted.
In the case where these compounds are not sufficiently water-soluble, they may be incorporated into the coating solutions by other common techniques known in the art. The compounds may for example be dissolved or emulsified in a mixture of water and water-soluble or water miscible organic solvents such as lower mono- or bis-alcohols, ketones, esters or amides.
Colloidal silicium dioxide, colloidal aluminium oxide or colloidal aluminium oxide/hydroxide may be used as porous inorganic oxide. Colloidal aluminium oxide or colloidal aluminium oxide/hydroxide are preferred. Especially preferred as colloidal aluminium oxide is y-Al2O3 and as colloidal AlOOH an AlOOH modified with salts of the rare earth metal series as described in patent application EP 0,875,394. This porous aluminium oxide/hydroxide contains one or more elements of the rare earth metal series of the periodic system of the elements with atomic numbers 57 to 71, preferably in a quantity from 0.4 to 2.5 mole percent relative to Al2O3. Especially preferred as aluminium oxide/hydroxide is pseudo-bohemite, an agglomerate of aluminium oxide/hydroxide of formula Al2O3xc2x7n H2O where n is from 1 to 1.5, or pseudo-bohemite modified with the salts of the rare earth metal series as also described in patent application EP 0,875,394. This porous pseudo-bohemite contains one or more elements of the rare earth metal series of the periodic system of the elements with atomic numbers 57 to 71, preferably in a quantity from 0.4 to 2.5 mole percent relative to Al2O3.
The binders are in most cases water-soluble polymers. Especially preferred are film forming polymers.
The water soluble polymers include for example natural polymers or modified products thereof such as albumin, gelatin, casein, starch, gum arabicum, sodium or potassium alginate, hydroxyethyl cellulose, carboxymethyl cellulose, xcex1-, xcex2- or xcex3-cyclodextrine and the like. In the case where one of the water-soluble polymers is gelatin, all known types of gelatin may be used as for example acid pigskin or limed bone gelatin, acid or base hydrolyzed gelatin, but also derivatised gelatins like for instance phthalaoylated, acetylated or carbamoylated gelatin or gelatin derivatised with the anhydride of trimellytic acid.
A preferred natural binder is gelatin.
Synthetic binders may also be used and include for example polyvinyl alcohol, polyvinyl pyrrolidone, completely or partially saponified products of copolymers of vinyl acetate and other monomers; homopolymers or copolymers of unsaturated carboxylic acids such as (meth)acrylic acid, maleic acid, crotonic acid and the like; homopolymers or copolymers of sulfonated vinyl monomers such as vinylsulfonic acid, styrene sulfonic acid and the like. Furthermore homopolymers or copolymers of vinyl monomers of (meth)acrylamide; homopolymers or copolymers of other monomers with ethylene oxide; polyurethanes; polyacrylamides; water soluble nylon type polymers; polyesters; polyvinyl lactams; acrylamide polymers; substituted polyvinyl alcohol; polyvinyl acetals; polymers of alkyl and sulfoalkyl acrylates and methacrylates; hydrolyzed polyvinyl acetates; polyamides; polyvinyl pyridines; polyacrylic acid; copolymers with maleic anhydride; polyalkylene oxides; methacrylamide copolymers and maleic acid copolymers may be used. All these polymers may also be used as mixtures.
Preferred synthetic binders are polyvinyl alcohol and polyvinyl pyrrolidone or mixtures thereof.
These polymers may be blended with water insoluble natural or synthetic high molecular weight compounds, particularly with acrylate latices or with styrene acrylate latices.
Although not specifically claimed in this invention water insoluble polymers are nevertheless considered part of the system.
The polymers mentioned above having groups with the possibility to react with a cross-linking agent can be cross-linked or hardened to form essentially water insoluble layers. Such cross-linking bonds may be either covalent or ionic. Cross-linking or hardening of the layers allows for the modification of the physical properties of the layers, like for instance in water absorption of the layer or in resistance against layer damage.
The cross-linking agents or hardeners are selected depending on the type of the water-soluble polymers used.
Organic cross-linking agents and hardeners include for example aldehydes (such as formaldehyde, glyoxal or glutaraldehyde), N-methylol compounds (such as dimethylol urea or methylol dimethylhydantoin), dioxanes (such as 2,3-dihydroxydioxane), reactive vinyl compounds (such as 1,3,5-trisacrylolyl hexahydro-s-triazine or bis-(vinylsulfonyl)methyl ether), reactive halogen compounds (such as 2,4-dichloro-6-hydroxy-s-triazine), epoxides, aziridines, carbamoyl pyridinium compounds or mixtures of two or more of the above mentioned cross-linking agents.
Inorganic cross-linking agents or hardeners include for example chromium alum, aluminium alum or boric acid.
The layers may also contain reactive substances that cross-link the layers under the influence of ultraviolet light, electron beams, X-rays or heat.
The layers may be modified by the addition of fillers. Possible fillers are for instance kaolin, Ca- or Ba-carbonates, silicium dioxide, titanium dioxide, bentonites, zeolites, aluminium silicate, calcium silicate or colloidal silicium dioxide. Likewise, the possibility exists to use organic inert particles such as polymer beads. These beads may consist of polyacrylates, polyacrylamides, polystyrene or different copolymers of acrylates and styrene. The fillers are selected according to the intended use of the printed images. Some of these compounds cannot be used if the printed images are to be used as transparencies. However they are of interest in cases where the printed images are be to used as remission pictures. Very often, the introduction of such fillers causes a wanted matte surface.
The recording sheets may further contain in addition water-soluble metal salts, as for example salts of the alkaline earths or salts of the rare earth metal series.
The recording sheets according to the invention comprise a support having coated thereon at least one ink-receiving layer, and, optionally, auxiliary layers.
A wide variety of supports are known and commonly used in the art. They include all those supports used in the manufacture of photographic materials. This includes clear films made from cellulose esters such as cellulose triacetate, cellulose acetate, cellulose propionate or cellulose acetate/butyrate, polyesters such as polyethylene terephthalate or polyethylene naphthalate, polyamides, polycarbonates, polyimides, polyolefins, polyvinyl acetals, polyethers, polyvinyl chloride and polyvinylsulfones. Polyester film supports, and especially polyethylene terephthalate or polyethylene naphthalate are preferred because of their excellent dimensional stability characteristics. The usual supports used in the manufacture of opaque photographic materials may be used including for example baryta paper, polyolefin coated papers, voided polyester as for instance manufactured by DuPont under the trade name of Melinex(copyright). Especially preferred are resin-coated paper or voided polyester.
When such support materials, in particular polyester, are used, a subbing layer is advantageously coated first to improve the bonding of the ink-receiving layers to the support. Useful subbing compositions for this purpose are well known in the photographic industry and include for example terpolymers of vinylidene chloride, acrylonitrile and acrylic acid or of vinylidene chloride, methyl acrylate and itaconic acid.
Also used as supports are plain paper, comprising all different types of papers varying widely in their composition and in their properties. Pigmented papers and cast-coated papers may also be used, as well as metal foils, such as foils made from aluminium.
The layers may also be coated onto textile fiber materials consisting of polyamides, polyesters, cotton, viscose, or wool.
The additive according to the invention may be added to any layer of the recording sheet.
The ink-receiving layers according to the invention are in general coated from aqueous solutions or dispersions containing all necessary ingredients. In many cases, surfactants are added to those coating solutions in order to improve the coating behavior and the evenness of the layers. Besides being necessary for coating purposes, these compounds may have an influence on the image quality and may therefore be selected with this specific goal in mind. Although not specifically claimed in this invention surfactants nevertheless form an important part of the invention.
In addition to the above mentioned compounds recording sheets according to the invention may contain additional compounds aimed at further improving their performance, as for example brightening agents to improve the whiteness, such as stilbenes, coumarines, triazines, oxazoles or others compounds known to someone skilled in the art.
Light stability can be improved by adding UV absorbers such as 2-hydroxybenzotriazoles, 2-hydroxybenzophenones, triazine derivatives or derivatives of cinnamic acids. The amount of UV absorber may vary from 200 mg/m2 to 2000 mg/m2, preferably from 400 mg/m2 to 1000 mg/m2. The UV absorber may be added to any of the layers of the recording sheet according to the invention. It is preferred however, if it is added, it should be added to the topmost layer.
It is further known that images produced by ink jet printing may be protected from degradation by the addition of radical scavengers, stabilizers, reducing agents and antioxidants. Examples of such compounds are sterically hindered phenols, sterically hindered amines, chromanols, ascorbic acid, phosphinic acid and its derivatives, sulfur containing compounds such as sulfides, mercaptans, thiocyanates, thioamides or thioureas.
The above-mentioned additives may be added as aqueous solutions to the coating solutions. In the case where these compounds are not sufficiently water-soluble, they may be incorporated into the coating solutions by other common techniques known in the art. The compounds may for example be dissolved in a water miscible solvent such as lower alcohols, glycols, ketones, esters, or amides. Alternatively, the compounds may be added to the coating solutions as fine dispersions, as oil emulsions, as cyclodextrine inclusion compounds or incorporated into latex particles.
Typically, the recording sheet according to the invention has a thickness in the range of 0.5 xcexcm to 100 xcexcm dry thickness, preferably in the range of 5 xcexcm to 50 xcexcm dry thickness.
The coating solutions may be coated onto the support by any number of suitable procedures. Usual coating methods include for example extrusion coating, air knife coating, doctor blade coating, cascade coating, or curtain coating. The coating solutions may also be applied using spray techniques. The ink-receiving layers may be built up from several single layers that can be coated one after the other or simultaneously. It is likewise possible to coat a support on both sides with ink-receiving layers. It is also possible to coat an antistatic layer or an anticurl layer on the backside. The selected coating method however is not to be considered limiting for the present invention.
It is also possible to immerse a recording sheet in a solution of the unsubstituted or substituted 1,3-cyclohexanedione in order to add the unsubstituted or substituted 1,3-cyclohexanedione to the recording sheet.
Inks for ink jet printing consist in essence of a liquid vehicle and a dye or pigment dissolved or suspended therein. The liquid vehicle for ink jet inks consists in general of water or a mixture of water and a water miscible organic solvent such as ethylene glycol, higher molecular weight glycols, glycerol, dipropylene glycol, polyethylene glycol, amides, polyvinyl pyrrolidone, N-methylpyrrolidone, cyclohexylpyrrolidone, carboxylic acids and their esters, ethers, alcohols, organic sulfoxides, sulfolane, dimethylformamide, dimethylsulfoxide, cellosolve, polyurethanes, acrylates and the like.
The non-aqueous parts of the ink generally serve as humefactants, cosolvents, viscosity regulating agents, ink penetration additives or drying agents. The organic compounds have in most cases a boiling point, which is higher than that of water. In addition, aqueous inks used for printers of the continuous stream type may contain inorganic or organic salts to increase their conductivity. Examples of such salts include nitrates, chlorides, phosphates, and salts of water-soluble organic acids such as acetates, oxalates, and citrates. The dyes and pigments suitable for the preparation of inks useable with the recording sheets according to the invention cover practically all classes of known coloring compounds. Dyes or pigments typically used for this purpose are described in patent application EP 0,559,324. The recording sheets according to this invention are meant to be used in conjunction with most of the inks representing the state of the art.
Other additives present in inks are for instance surfactants, optical brighteners, UV absorbers, light stabilizers, biocides, precipitating agents such as multivalent metal compounds and polymeric additives.
This description of inks is for illustration only and is not to be considered as limiting for the purpose of the invention.
The present invention will be illustrated in more detail by the following examples without limiting the scope of the invention in any way.